In the drilling of oil and gas wells, it is common to drive the drill bit by a downhole mud motor located at the end of a drill string. In particular, drilling fluid, generally referred to as drill mud, is circulated to drive the motor by positive hydraulic displacement or turbine action. The mud then passes through the ports in the drill bit and carries material cut by the drill bit back to the surface through the annular space between the drill pipe and the resulting bore hole.
Bearing assemblies for wellbore drilling are mounted between the drill bit and the mud motor to permit rotation of the drill bit. The drill bit is attached to a hollow drive shaft, also known as a mandrel that is located within a bearing housing. The mandrel is rotated by the mud motor while the bearing housing is fixed to the drill string and remains relatively stationary. In its position behind the drill bit, the bearing assembly is subject to significant radial and axial loading. Radial and thrust bearings are thus located along the bearing assembly to absorb radial and axial loads.
It is often desired to have a borehole which deviates from the vertical or from an initial section of the well, in the case of slant drilling. Bent housings have been developed to permit such directional drilling. Conventional adjustable bent housings use an adjustable mandrel which provides a connection to the stator housing of the drilling motor, and a box connection to the bottom end of the bearing assembly. As is well known, the drive shaft passes through the bent housing.
Adjustable bent housings provide axial deviations by machining components of the housing to create a deviation, by machining deviating threads, or by machining one end of a tubular body which deviates from the other end. By combining two tubular components with axial deviations, an adjustable housing may vary between a straight configuration, and a cumulative bent configuration, which is the sum of the two deviations. These two components are threaded together and a locking nut or adjusting nut is provided to fix the two components together with the desired deviation. This locking nut must have a left-hand thread or a thread opposite the threaded connection of the two main tubular components.
Prior art adjustable bent housings often have two offset points, which commonly results in an oversize wellbore because a true straight housing cannot be achieved. In other cases, adjustable bent housings with a single offset point are known. However, in each case, the offset point is located above the bearing assembly, in the motor housing.